MEMS device with off-axis actuator

ABSTRACT

A micro-electro-mechanical system (MEMS) mirror device has a mirror, a frame rotatively coupled to the mirror, and a uniaxial actuator rotatively coupled to the frame where the rotational axis of the actuator is offset from the rotational axes of the mirror and the frame. Another MEMS mirror device has a mirror, a frame rotatively coupled to the mirror, and a biaxial actuator rotatively coupled to the frame where the actuator is able to rotate about the rotational axes of the mirror and the frame with the mirror.

FIELD OF INVENTION

This invention relates to micro-electro-mechanical system (MEMS)devices, and more particularly to MEMS scanning mirrors.

DESCRIPTION OF RELATED ART

U.S. Pat. Nos. 6,769,616 and 7,034,370 disclose a bidirectional scanningMEMS mirror system. In the system, a mirror is rotatively coupled to aframe and the frame is rotatively mounted to an anchor layer. Actuatorsthat consist of electrodes extending from the outer perimeter of themirror and the inner perimeter of the frame rotate the mirror about afirst axis. Actuators that consist of electrode extending from the outerperimeter of the frame and the inner perimeter of stationary pads rotatethe frame about a second axis. The result is a rather complicated designof the mirror and the frame that allows for rotation and electricalisolation of the voltages necessary to rotate the mirror about the firstaxis. Thus, what is needed is a simplified design for a bidirectionalscanning MEMS mirror system.

SUMMARY

In embodiments of the invention, a micro-electro-mechanical system(MEMS) mirror device has a mirror, a frame rotatively coupled to themirror, and a uniaxial actuator rotatively coupled to the frame wherethe rotational axis of the actuator is offset from the rotational axesof the mirror and the frame. This configuration allows a single uniaxialactuator to provide two axes of motion to the mirror, therebysimplifying the design of the device.

In other embodiments of the invention, a MEMS mirror device has amirror, a frame rotatively coupled to the mirror, and a biaxial actuatorrotatively coupled to the frame where the actuator is able to rotateabout the rotational axes of the mirror and the frame with the mirror.This configuration allows a single biaxial actuator to provide two axesof motion to the mirror, thereby simplifying the design of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a micro-electro-mechanical system (MEMS) mirror device in oneembodiment of the invention.

FIG. 2 is a MEMS mirror device in one embodiment of the invention.

FIG. 3 is a MEMS mirror device in one embodiment of the invention.

Use of the same reference numbers in different figures indicates similaror identical elements.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a micro-electro-mechanical system (MEMS) mirrordevice 100 in one embodiment of the invention. Device 100 is typicallymade from a silicon substrate using conventional semiconductorprocessing methods (e.g., masking and etching). Device 100 can be usedin any application that requires one or two axes of motion (e.g., aunidirectional or a bidirectional scanning mirror).

Device 100 includes a mirror 102 connected by springs 104 and 106 to aframe 108. The coupling between mirror 102 and springs 104 and 106 arelocated along a rotational axis 110 (hereafter mirror axis 110) so themirror can rotate about the mirror axis relative to frame 108. In oneembodiment, a rectangular mirror 102 is located within a rectangularframe 108, where the left and the right sides of the mirror areconnected by springs 104 and 106 to the left and the right sections ofthe frame, respectively. Mirror 102 and frame 108 may have other shapesin other embodiments. Note that any use of direction and orientation isfor illustrative purposes and is not intended to limit the actualdirection and orientation of device 100 in use.

Frame 108 is connected by springs 112 and 114 to a uniaxial actuator116. The coupling between frame 108 and springs 112 and 114 are locatedalong a rotational axis 118 (hereafter frame axis 118) so the frame canrotate about the frame axis relative to actuator 116. Frame axis 118 isoffset from mirror axis 110 so effectively mirror 102 has two axes ofrotation.

Actuator 116 includes a left portion 116L and a right portion 116R(hereafter actuator portion 116L and actuator portion 116R). Actuatorportion 116L is connected by one or more springs 124 to one or morestationary spring pads 126. Similarly, actuator portion 116R isconnected by one or more springs 128 to one or more stationary springpads 130. Stationary spring pads 126 and 130 are typically mounted to asubstrate. For illustrative purposes, only four stationary spring padsare shown where two are located in openings in actuator portions 116Land 116R, and two are located at the ends of the actuator portions. Thecoupling between actuator 116 and springs 124 and 128 are located alonga rotational axis 136 (hereafter actuator axis 136) so the actuator canrotate about the actuator axis. Actuator portions 116L and 116R arejoined by top and bottom beams 116T and 116B so the actuator portionsrotate in unison to drive frame 108 and mirror 102. Beams 116T and 116Bare connected by springs 112 and 114 to the top and the bottom sectionsof frame 108, respectively.

To allow an uniaxial actuator 116 to provide two axes of motion tomirror 102, actuator axis 136 is offset from mirror axis 110 and frameaxis 118. This configuration divides the torque generated by actuator116 along actuator axis 136 into (1) a first torque component thatrotates mirror 102 along mirror axis 110 and (2) a second torquecomponent that rotates frame 108 along frame axis 118.

To achieve large rotation angles for mirror 102 and frame 108, aresonant frequency of the mirror about mirror axis 110 and a resonantfrequency of the frame with the mirror about frame axis 118 are setequal to the scanning frequencies needed along two axes in theapplication of device 100. In one embodiment, the resonant frequency ofmirror 102 is different than the resonant frequency of frame 108 withthe mirror by at least 50% so the rotations about mirror axis 110 andframe axis 118 can be individually controlled. Typical range for theratio of the two resonant frequencies is 1.2 to 50. Computer modelingcan be used to determine the resonant frequencies of different designsfor mirror 102 and frame 108.

When the resonant frequency of mirror 102 is higher than the resonantfrequency of frame 108, the angle formed between actuator axis 136 andmirror axis 110 is smaller than the angle formed between the actuatoraxis and frame axis 118. This is because it takes more energy to excitemirror 102 as it has a higher resonant frequency so actuator axis 136needs to be more closely aligned to mirror axis 110 than frame axis 118.The actual angles of a design depend on the resonant frequencies andother factors, including the scan angles and the moment of inertia.Typical angles may range from 1 to 40 degrees.

Actuator 116 resonantly oscillates frame 108 and mirror 102 with amotion including (1) a first oscillation with a first amplitude and afirst frequency, and (2) a second oscillation with a second amplitudeand a second frequency superimposed on the first oscillation. The firstfrequency is set equal to the resonant frequency of frame 108 withmirror 102 and the second frequency is set equal to the resonantfrequency of the mirror, or vice versa. For example, actuator 116 has amotion including a large but slow oscillation that excites frame 108with mirror 102 about frame axis 118. Along the path of the large butslow oscillation, the motion further includes a small but fastoscillation that excites mirror 102 about mirror axis 110. Theoscillation of mirror 102 about mirror axis 110 can be amplified throughthe elastic spring coupling generally along the mirror axis.

In one embodiment, actuator 116 is an electrostatic actuator. Actuator116 includes movable electrodes 137L and 137R (only one of each islabeled; collectively referred to as movable electrode 137) that extendfrom the top edges of actuator portions 116L and 116R, respectively, andmovable electrodes 138L and 138R (only one of each is labeled;collectively referred to as movable electrode 138) that extend from thebottom edges of actuator portions 116L and 116R, respectively. Movableelectrodes 137L and 137R are interdigitated out of plane with stationaryelectrodes 140L and 140R (only one of each is labeled; collectivelyreferred to as stationary electrode 140), respectively. Stationaryelectrodes 140L and 140R extend from stationary electrode pads 142L and142R (collectively referred to as stationary electrode pad 142),respectively. Note that pads 142L and 142R may be replaced with a singlepad. Movable electrodes 138L and 138R are interdigitated out of planewith stationary electrodes 144L and 144R (only one of each is labeled;collectively referred to as stationary electrode 144), respectively.Stationary electrodes 114L and 114R extend from stationary electrodepads 146L and 146R (collectively referred to as stationary electrode pad146), respectively. Note that pads 146L and 146R may be replaced with asingle pad. The stationary electrodes and the stationary electrode padsare typically made from a layer above or below mirror 102, frame 108,and actuator 116.

In one embodiment, voltage sources 148, 150, and 152 are respectivelycoupled to actuator 116, stationary electrode pad 142, and stationaryelectrode pad 146. Voltage sources 148 and 150 can supply a periodicvoltage difference between electrodes 137 and 140 to oscillate actuator116 about actuator axis 136. Similarly, voltage sources 148 and 152 cansupply a periodic voltage difference between electrodes 138 and 144 tooscillate actuator 116 about actuator axis 136. The two periodic voltagedifferences can be substantially out of phase (e.g., by 180 degrees) towork together to oscillate actuator 116.

In one embodiment, voltage source 148 provides a steady voltage (e.g.,ground) to electrodes 137 and 138, voltage source 150 provides a voltagehaving a first waveform to electrodes 140, and voltage source 152provides a voltage having a second waveform to electrodes 144. The firstwaveform has a first oscillating signal with a first amplitude and afirst frequency, and a second oscillating signal with a second amplitudeand a second frequency superimposed on the first signal. The secondwaveform is a complement of the first waveform that is substantially outof phase (e.g., by 180 degrees) with the first waveform. The firstfrequency is set equal to the resonant frequency of frame 108 withmirror 102 and the second frequency is set equal to the resonantfrequency of the mirror, or vice versa. For example, the waveform has afirst square wave signal with large amplitude but low frequency, and asecond square wave signal with small amplitude but high frequencysuperimposed on the first square wave signal.

FIG. 2 illustrates a MEMS mirror device 200 in one embodiment of theinvention. Device 200 is similar to device 100 except the rotationalaxis 218 (hereafter frame axis 218) of frame 108 is less offset fromactuator axis 136 than the rotational axis 210 (hereafter mirror axis210) of mirror 102. In this embodiment, the top and the bottom sides ofmirror 102 are connected by springs 204 and 206 to the top and thebottom sections of frame 108, respectively, and the left and the rightsections of the frame are connected by springs 212 and 214 to actuatorportions 116L and 116R, respectively. The coupling between mirror 102and springs 204 and 206 are placed along mirror axis 210, and thecoupling between frame 108 and springs 212 and 214 are placed alongframe axis 218. Also note that a single stationary electrode pad 142with stationary electrodes 140 is shown, and a single stationaryelectrode pad 146 with stationary electrodes 144 are shown.

In contrast to the arrangement of device 100, actuator axis 136 is now(1) slightly offset from frame axis 218 so they are not parallel and (2)largely offset from mirror axis 210 but they are not orthogonal. Thisconfiguration divides the torque generated by actuator 116 alongactuator axis 136 into (1) a first larger torque component that rotatesframe 108 with mirror 102 along frame axis 218, and (2) a second smallertorque component that rotates mirror 102 along mirror axis 210. Device200 is driven in the same manner as device 100.

FIG. 3 illustrates a MEMS mirror device 300 in one embodiment of theinvention. Device 300 is similar to device 100 except actuator 116 nowoperates as a biaxial actuator that rotates about axes 136 and 318,mirror 102 rotates about axis 136, and frame 108 rotates axis 318. Axes136 and 318 are offset and they are typically orthogonal to each other.

Actuator 116 can resonantly oscillate mirror 102 with a firstoscillation about axis 136 at a first frequency. The first frequency isset equal to the resonant frequency of mirror 102 about axis 136.Actuator 116 can resonantly oscillate frame 108 with mirror 102 with asecond oscillation about axis 318 at a second frequency. The secondfrequency is set equal to the resonant frequency of frame 108 withmirror 102 about frame axis 318. Actuator 116 can resonantly oscillateboth mirror 102 and frame 108 with the mirror by providing the first andthe second oscillations at the same time. The amplitudes of the firstand the second oscillations are different. The oscillation of mirror 102about axis 136 can be amplified through the elastic spring couplinggenerally along axis 136.

In one embodiment, voltage sources 148, 150L, 150R, 152L, and 152R arerespectively coupled to actuator 116, stationary electrode pad 142L,stationary electrode pad 142R, stationary electrode pad 146L, andstationary electrode pad 146R. To rotate actuator 116 about axis 136,voltage sources 148, 150L, and 150R can supply a periodic voltagedifference between electrodes 137 and electrodes 140. Similarly, voltagesources 148, 152L, and 152R can supply a periodic voltage differencebetween electrodes 138 and electrodes 144. The two periodic voltagedifferences can be substantially out of phase (e.g., by 180 degrees) towork together to oscillate actuator 116 about axis 136.

To rotate actuator 116 about axis 318, voltage sources 148, 150L, and152L can supply a periodic voltage difference between (1) electrodes137L/138L and (2) electrodes 140L/144L. Similarly, voltage sources 148,150R, and 152R can supply a periodic voltage difference between (1)electrodes 137R/138R and (2) electrodes 140R/144R. The two periodicvoltage differences can be substantially out of phase (e.g., by 180degrees) to work together to oscillate actuator 116 about axis 318.

In one embodiment, the following table lists voltages for causingappropriate actuator motions. Signal V1 is a first oscillating signalhaving a first amplitude and a first frequency for causing oscillationabout axis 318, signal V2 is a second oscillating signal having a secondamplitude and a second frequency for causing oscillation about axis 136,signal V4 is a complement of signal V1 with the same amplitude and asubstantial phase offset in the frequency (e.g., 180 degree offset), andsignal V3 is a complement of signal V2 with the same amplitude but asubstantial phase offset in the frequency (e.g., 180 degree offset). Thefirst frequency is set equal to the resonant frequency of frame 108 withmirror 102 and the second frequency is set equal to the resonantfrequency of the mirror, or vice versa. The first and the secondamplitudes of the oscillating signals are different. For example, signalV1 is a first square wave signal with large amplitude but low frequency,and signal V2 is a second square wave signal with small amplitude buthigh frequency.

TABLE Voltage source Signals (V) 148 Steady (e.g., ground) 150L V1 + V2150R V4 + V2 152L V1 + V3 152R V4 + V3

Various other adaptations and combinations of features of theembodiments disclosed are within the scope of the invention. Althoughrectangular mirror 102 and frame 108 are described, the mirror and theframe may be round, polygonal, or another suitable shape. Althoughstraight springs are shown, the springs may be serpentine or anothersuitable shape. Although electrostatic actuator 116 is described, theactuator can be electromagnetic, piezoelectric, or another suitabletechnology. Although voltages having the waveform of square waves aredescribed, the waveform can be sinusoidal, triangular, sawtooth, oranother suitable waveform, Numerous embodiments are encompassed by thefollowing claims.

1. A micro-electro-mechanical system (MEMS) mirror device, comprising: amirror; a frame rotatively coupled to the mirror so the mirror isrotatable about a first axis relative to the frame, the mirror has afirst resonant frequency for oscillating about the first axis; anactuator rotatively coupled to the frame so the frame is rotatable abouta second axis relative to the actuator, the second axis being offsetfrom the first axis, the frame with the mirror has a second resonantfrequency for oscillating about the second axis; and stationary springpads rotatively coupled to the actuator so the actuator is rotatableabout a third axis, the actuator being operable to rotate only about thethird axis, the third axis being offset from the first and the secondaxes.
 2. The device of claim 1, wherein a first angle between the firstand the third axes is smaller than a second angle between the second andthe third axes.
 3. The device of claim 1, wherein a first angle betweenthe first and the third axes is larger than a second angle between thesecond and the third axes.
 4. The device of claim 1, wherein the firstand the second axes are perpendicular.
 5. The device of claim 1, whereinthe actuator is operable to oscillate in a motion comprising (1) a firstoscillation with a first frequency and (2) a second oscillation with asecond frequency superimposed on the first oscillation, the firstfrequency being equal to one of the first and the second resonantfrequencies, and the second frequency being equal to the other one ofthe first and the second resonant frequencies.
 6. The device of claim 5,wherein amplitudes of the first and the second oscillations aredifferent.
 7. The device of claim 1, wherein: the actuator comprises afirst portion extending from a first proximate end near the frame to afirst distal end spaced from the frame along the third axis, the firstportion comprising first movable electrodes and second movableelectrodes that are located on the opposite sides of the third axis; andthe device further comprises: first stationary electrodes interdigitatedout of plane with the first movable electrodes; and secondary stationaryelectrodes interdigitated out of plane with the second movableelectrodes.
 8. The device of claim 7, further comprising voltage sourcescoupled to the actuator, the first stationary electrodes, and the secondstationary electrodes to generate (1) a first periodic voltagedifference between the first movable and the first stationary electrodesand (2) a second periodic voltage difference between the second movableand the second stationary electrodes, wherein: the first periodicvoltage difference being a first waveform with (1) a first oscillatingsignal with a first frequency and (2) a second oscillating signal with asecond frequency superimposed on the first oscillating signal, the firstfrequency being equal to one the first and the second resonantfrequencies, and the second frequency being equal to the other one ofthe first and the second resonant frequencies; and the second periodicvoltage difference being a second waveform that is a complement of thefirst waveform that is substantially out of phase with the firstwaveform.
 9. The device of claim 8, wherein amplitudes of the first andthe second oscillating signals are different.
 10. The device of claim 8,wherein: the actuator further comprises: a second portion extending froma second proximate end near the frame to a second distal end spaced fromthe frame along the third axis, the first and the second portions beinglocated on different sides of the frame, the second portion comprisingthird movable electrodes and fourth movable electrodes that are locatedon the opposite sides of the third axis; and top and bottom beams thatconnect the first and the second portions so they rotate in unison, thetop and the bottom beams being rotatively coupled to the frame; and thedevice further comprises: third stationary electrodes interdigitated outof plane with the third movable electrodes; and fourth stationaryelectrodes interdigitated out of plane with the fourth movableelectrodes.
 11. The device of claim 10, wherein the voltage sources arefurther coupled to the third stationary electrodes and the fourthstationary electrodes to generate (1) a third periodic voltagedifference between the third movable and the third stationary electrodesand (2) a fourth periodic voltage difference between the fourth movableand the fourth stationary electrodes, wherein: the third periodicvoltage difference being the first waveform; and the fourth periodicvoltage difference being the second waveform.
 12. A method for operatinga micro-electro-mechanical system (MEMS) mirror device comprising (1) amirror, (2) a frame rotatively coupled to the mirror so the mirror isrotatable about a first axis relative to the frame, (3) an actuatorrotatively coupled to the frame so the frame is rotatable about a secondaxis relative to the actuator, and (4) stationary spring pads rotativelycoupled to the actuator so the actuator is rotatable, the mirror havinga first resonant frequency, the frame with the mirror having a secondresonant frequency, the method comprising: oscillating the actuator onlyabout a third axis offset from the first and the second axes in a motioncomprising (1) a first oscillation with a first frequency and (2) asecond oscillation with a second frequency superimposed on the firstoscillation, the first frequency being equal to one of the first and thesecond resonant frequencies, and the second frequency being equal to theother one of the first and the second resonant frequencies.
 13. Themethod of claim 12, wherein a first angle between the first and thethird axes is smaller than a second angle between the second and thethird axes.
 14. The method of claim 12, wherein a first angle betweenthe first and the third axes is larger than a second angle between thesecond and the third axes.
 15. The method of claim 12, wherein the firstand the second axes are perpendicular.
 16. The method of claim 12,wherein amplitudes of the first and the second oscillations aredifferent.
 17. The method of claim 12, wherein said oscillating theactuator comprises providing a first periodic voltage difference,wherein the first periodic voltage difference being a first waveformwith (1) a first oscillating signal with a first frequency and (2) asecond oscillating signal with a second frequency superimposed on thefirst oscillating signal, the first frequency being equal to one of thefirst and the second resonant frequencies, and the second frequencybeing equal to the other one of the first and the second resonantfrequencies.
 18. The method of claim 17, wherein said oscillating theactuator further comprises providing a second periodic voltagedifference, the second periodic voltage difference being a secondwaveform that is a complement of the first waveform that issubstantially out of phase with the first waveform.
 19. The method ofclaim 18, wherein amplitudes of the first and the second oscillatingsignals are different.